1. Field of the Invention
The present invention relates to an apparatus for non-invasively estimating the waveform of blood pressure in a central artery (e.g., an aortic artery or a carotid artery) of a living subject.
2. Discussion of Related Art
There is known a blood-pressure (BP) estimating device which continuously estimates, by so-called tonometry, BP of a living subject, such as a patient under a surgical operation, to monitor his or her BP. The BP estimating device includes a pressure-pulse-wave (PPW) sensor which is pressed via skin against a peripheral artery, such as a radial artery, to detect a peripheral PPW from the artery, and continuously estimates the BP of the subject based on the detected peripheral PPW.
However, the waveform of the peripheral PPW detected by the PPW sensor differs from the waveform of blood pressure in a central artery, in that the waveform of peripheral PPW is distorted as compared with the waveform of central-artery pressure and is time-wise delayed from the same. Therefore, if the waveform of central-artery pressure is continuously estimated based on the waveform of peripheral PPW by the BP estimating device, then it would be necessary to correct the waveform of peripheral PPW in an appropriate manner.
There has been proposed a method of correcting the distortion of the waveform of peripheral PPW to estimate continuously the waveform of central aortic pressure (i.e., blood pressure at the heart-side end of aorta) as a sort of central-artery pressure. This method includes determining an average transfer function between the waveform of central aortic pressure and the waveform of peripheral PPW, and correcting the continuously detected peripheral PPW based on the transfer function. In addition, there has been also proposed a method of estimating the waveform of central aortic pressure by (a) separating the waveform of peripheral PPW into an estimated waveform of forward PPW and an estimated waveform of backward PPW, (b) estimating, based on the estimated waveforms of forward and backward PPWs and a propagation time in which the PPW propagates between the heart-side end of aorta and a portion of the peripheral artery against which the PPW sensor is pressed, the respective waveforms of forward and backward PPWs at the heart-side end of aorta, and (c) estimating the waveform of central aortic pressure by adding the estimated waveforms of forward and backward PPWs at the heart-side end of aorta. (Stergiopulos N, Westerhof B E, Westerhof N: Physical basis of pressure transfer from periphery to aorta; a model-based study. American Journal of Physiology 1998; 274; H1386-H1392).
However, since the above-indicated former or first method needs complex calculations including Fourier transformation and/or convolution integration, it is difficult to monitor the waveform of central aortic pressure on a real-time basis. In addition, though the latter or second method can accurately estimate, by simple calculations, the waveform of central aortic pressure, it has the disadvantage that it needs to measure blood flow in the peripheral artery.
Moreover, there has been proposed a third method of determining a transfer function based on a vascular-system model, and estimating the waveform of central aortic pressure based on the transfer function and BP measured from a finger. (Karamanoglu M, Feneley M P: On-line synthesis of the human ascending aortic pressure pulse from the finger pulse. Hypertension. 1997; 30; 1416-1424).
However, the third method has the disadvantage that it needs a parameter which can be measured in an invasive manner only.
It is therefore an object of the present invention to provide an apparatus which can estimate, with each and accuracy, a waveform of blood pressure in a central artery of a living subject.
The above object has been achieved by the present invention, which provides an apparatus for non-invasively estimating a waveform of a blood pressure in a central artery of a living subject, comprising a pressure-pulse-wave detecting device which includes a pressure-pulse-wave sensor adapted to be pressed, via a skin of the subject, against a first portion of a peripheral artery located on a downstream side of the central artery and which non-invasively detects, through the pressure-pulse-wave sensor, a pressure pulse wave produced from the first portion of the peripheral artery; a blood-pressure-difference determining means for determining, according to a predetermined vascular-system model, a blood-pressure difference between a blood pressure at the first portion of the peripheral artery pressed by the pressure-pulse-wave sensor and a blood pressure at an end of the peripheral artery, based on the pressure pulse wave detected by the pressure-pulse-wave detecting device; a peripheral-artery-blood-pressure-waveform estimating means for estimating, based on the pressure pulse wave detected by the pressure-pulse-wave detecting device and the blood-pressure difference determined by the blood-pressure-difference determining means, a waveform of a forward pressure pulse wave at the first portion of the peripheral artery pressed by the pressure-pulse-wave sensor, and a waveform of a backward pressure pulse wave at the first portion of the peripheral artery; a propagation-time determining means for determining a propagation time in which the pressure pulse wave propagates from a second portion of the central artery to the first portion of the peripheral artery; and a central-artery-blood-pressure-waveform estimating means for estimating, based on the respective waveforms of the forward and backward pressure pulse waves estimated by the peripheral-artery-blood-pressure-waveform estimating means and the propagation time determined by the propagation-time determining means, a waveform of a forward pressure pulse wave at the second portion of the central artery and a waveform of a backward pressure pulse wave at the second portion of the central artery, and estimating a waveform of a blood pressure at the second portion of the central artery, by adding the respective estimated waveforms of the forward and backward pressure pulse waves at the second portion of the central artery.
In the present apparatus, the blood-pressure-difference determining means determines, according to the predetermined vascular-system model, the blood-pressure difference between the blood pressure at the first portion of the peripheral artery pressed by the pressure-pulse-wave sensor and the blood pressure at the end of the peripheral artery, based on the pressure pulse wave non-invasively detected by the pressure-pulse-wave detecting device. In addition, the peripheral-artery-blood-pressure-waveform estimating means estimates, based on the pressure pulse wave detected by the pressure-pulse-wave detecting device and the blood-pressure difference determined by the blood-pressure-difference determining means, the waveform of forward pressure pulse wave at the first portion of the peripheral artery, and the waveform of backward pressure pulse wave at the first portion of the peripheral artery, and the central-artery-blood-pressure-waveform estimating means estimates, based on the respective waveforms of the forward and backward pressure pulse waves and the propagation time determined by the propagation-time determining means, the waveform of blood pressure in the central artery. Thus, the present apparatus can easily estimate the waveform of blood pressure in the central artery, by just detecting the pressure pulse wave from the peripheral artery and measuring the propagation time.